Evolution of Magma-plumbing System beneath Fuji Volcano : On the view point of whole-rock chemistry(<Special Section>Models of Volcano (II))

Abstract

Evolution of whole-rock chemistry of volcanic rocks in Fuji volcano is presented. Basaltic andesite is predominant during 66,000 to 80,000 y. B. P. ; they are poor in FeO^*, MgO, MnO, K₂O, TiO₂, P₂O₅, and abundant in Al₂O₃ and Na₂O, with slightly high K₂O/TiO₂, Rb/Y, and Zr/Y ratios. Basalts erupted during 22,000 to 66,000 y. B. P. show low FeO^*/MgO ratio (2.0＞), and are lacking in those with high K₂O, TiO₂, P₂O₅, Rb, Ba, and Zr, and low Al₂O₃ ; they are characterized by low K₂O/TiO₂, Rb/Y, and Zr/Y ratios. Eruption products during 10,000 to 22,000 y. B. P., especially in its later stage, are basalt with relatively high FeO^*/MgO, Rb/Y, and Zr/Y ratios, and K₂O, TiO₂, P₂O₅, Rb, Ba, and Zr contents, and low Al₂O₃ ; their chemistry resembles to those after 10,000 y. B. P.. Ejecta during 8,000 to 10,000 y. B. P. are basalt and high in FeO^*/MgO, Rb/Y, and Zr/Y ratios, and K₂O, TiO₂, P₂O₅, Rb, Ba, and Zr contents, and Rb/Y and Zr/Y ratios, and low in Al₂O₃. Volcanic rocks during 3,000 to 8,000 y. B. P. are basalt showing low K₂0/TiO₂, Rb/Y, Ba/Y, and Zr/Y ratios, and K₂O, TiO₂, P₂O₅, Rb, Ba, Y, and Zr contents, and high FeO^*/MgO and Al₂O₃ ; they are rather similar to those erupted during 22,000 to 66,000 y. P. B. in chemistry. Erupted materials during 2,000 to 3,000 y. B. P. are basalt with low K₂O, TiO₂, P₂O₅, Rb, Ba, and Zr contents, and Rb/Y and Zr/Y ratios, and high Al₂O₃ and FeO^*/MgO ; their chemistry are akin to those during 3,000 to 8,000 y. B. P., but slightly enriched in incompatible elements. Eruption products after 2,000 y. B. P. are abundant in K₂O, TiO₂, P₂O₅, Rb, Ba, and Zr, and poor in Al₂O₃, with high FeO^*/MgO, Rb/Y, and Zr/Y ratios ; they are almost similar to basalts erupted during 8,000 to 10,000 y. B. P.. The Fuji volcano is classified into three magma-series on the view point of whole-rock Rb/Y and Zr/Y ratios : Early Ko-Fuji (66,000 to 80,000 y. B. P.), Late Ko-Fuji (10,000 to 66,000 y. B. P.), and Shin-Fuji (present to 10,000 y. B. P.). Basaltic magmas of each series may have probably been derived successively from chemically heterogeneous upper mantle. The high rate of change of source rock chemistry may reflect dynamic process in the upper mantle beneath the arc volcano.